I feel that currently not many things in the DDbeta are obvious at first glance at how to unlock, so i just wanted to make a checklist.

Witch hut was unlocked by collecting stuff in a subdungeon in creeplight ruins.Gnomes and halflings were both found in a subdungeon in Venture Cave and Den of Danger respectively.Bazaar was found by paying 35gold to buy the scroll in any dungeon.All 4 gods were unlocked by completing their quest in the subdungeons in northern desert, western jungle, southern swamp and eastern tundra.

The Witch can be found in "any dungeon in the east", I think, because I unlocked her in Eastern Tundra.

The thing to remember is that it's random when her subdungeon appears. So you basically have to keep grinding those areas.

There's also the Orc race. I don't know if they appear specifically in certain locations but they'll show up in a random subdungeon like the witch and ask you to drink a tea that poisons and reduces your hp to 1 to get them.